Optical materials and optical products are useful to control the flow and intensity of light. Examples of useful optical products include optical lenses such as Fresnel lenses, prisms, optical light fibers, light pipes, optical films including totally internal reflecting films, retroreflective sheeting, and microreplicated products such as brightness enhancement films and security products. Examples of some of these products are described in U.S. Pat. Nos. 4,542,449, 5,175,030, 5,591,527, 5,394,255, among others.
Polymeric materials have found a variety of uses in optical articles and are widely used in place of such articles made from ground glass because the former are light in weight and inexpensive to produce. Polycarbonates, for example, are characterized by excellent clarity, resistance to discoloration, high strength, and high impact resistance. However, thermal polymerization of monomers to form polymers is generally accompanied by high shrinkage during cure (e.g., from 11 to 20%) and extended curing time (e.g., from 5 to 16 hours or more). The high shrinkage levels create difficulties in the production of precision optics (such as lenses or prisms) from this material, particularly in the production of articles having larger thicknesses or large differences in thickness between the center and edges of the article. The extended cure times tie up production facilities and lead to inefficient utilization of the dies in which the articles are molded. Also, the thermal cure cycle used to polymerize the monomer consumes large amounts of energy and undesirably thermally stresses the dies.
Optical products can be prepared from high index of refraction materials, including monomers such as high index of refraction (meth)acrylate monomers, halogenated monomers, etc., and other such high index of refraction monomers that are known in the optical product art. See, e.g., U.S. Pat. Nos. 4,568,445, 4,721,377, 4,812,032, and 5,424,339. Some of these polymers may be advantageously injection molded, but such molding operations lead to high birefringence in the resulting article, and a subsequent annealing step may be required. Further, poly(methyl methacrylate) polymers tend to be moisture sensitive, and will swell on exposure to moisture or humidity, further leading to birefringence.
Several disclosures are related to optical coatings, which are generally less than two mils (50.8 micrometers) thick. They fail to describe if those compositions would have a desired balance of useful properties such as low polymerization shrinkage, low viscosity, absence of coloration, high hardness, resistance to stress cracking, moisture or humidity sensitivity and low birefringence necessary in the production of precision optical components such as lenses, including Fresnel lenses, and prisms. Additionally, they fail to teach how to obtain resins providing the desired balance of properties that are useful for providing cast precision optical articles. Moreover, many of the polymeric compositions generally have too high a viscosity to be useful for optical casting purposes.